This invention relates to a method of manufacturing a field effect transistor, more particularly a method of manufacturing a field effect transistor device in which junction regions which constitute the field effect transistor are disposed in a compound semiconductor layer on a semiinsulating substrate made of a similar compound semiconductor.
An ordinary field effect transistor now being used widely is constructed such that an N type semiconductor layer made of such compound semiconductor as GaAs is epitaxially grown on a semiinsulating substrate made of the similar compound semiconductor; that a source electrode and a drain electrode having a predetermined spacing are attached to the surface of the semiconductor layer with ohmic contacts and that a gate electrode in a Schottky junction with the semiconductor layer is disposed between the source and drain electrodes. Such a construction is disclosed in a Charles A. Liechti's paper entitled "Microwave Field-Effect Transistor--1976", I.E.E.E. Transaction on Microwave Theory and Techniques, Vol. MTT-24, No. 6 pages 279-300, June, 1976.
In a transistor of this construction, a depletion layer extends into the semiconductor layer from the Schottky junction according to the magnitude of a control voltage impressed across the gate and source electrodes so that the cross-sectional area of a drain current path in the semiconductor layer is narrowed in accordance with the gate control voltage.
Moreover, the transistor constructed as above described involves the following problems.
Firstly, as the semiconductor layer is epitaxially grown on the semiinsulating substrate, in most cases, portions of the semiconductor layer near the substrate have crystal defects. In addition, since these portions are formed at the initial stage of epitaxial growth, the impurity concentration is difficult to make uniform due to manufacturing technique. For this reason it is extremely difficult to make uniform the characteristics of the transistor at or near cutting off the drain current (gate pinch off) regardless of an accurate control of the thickness of the semiconductor layer in the subsequent process for manufacturing steps of the transistor. Thus, this problem is one of the factors that decreases the yield of satisfactory transistors. This is more particularly true in transistors assembled into an integrated circuit. For example, it is desired to ON/OFF control the drain current with relatively small gate control voltage of about .+-.1 volt, in which case the thickness of the semiconductor layer becomes about 0.1 to 0.15 micron for 1.times.10.sup.17 to 5.times.10.sup.16 cm.sup.-3 of N type impurity concentration in GaAs so that the problem caused by the construction described above results in a large dispersion in the gate pinch off voltage.
For this reason, it is difficult to assemble transistors into an integrated circuit for commercial use.
Furthermore, the transistor of this type is manufactured by the steps of forming a semiconductive layer of one conductivity type by implanting ions of an impurity into one surface of a semiinsulating substrate made of such compound as GaAs, forming source and drain electrodes in an ohmic contact with the surface of the substrate, and then forming a gate electrode to form a Schottky junction. Transistors prepared by this method are disclosed in a B. M. Welch et al. paper entitled "Gallium Arsenide Field-Effect Transistors by Ion Implantation", Journal of Applied Physics, Vol. 45, No. 8, pages 3685-3687, August 1974 and a R. G. Hunsperger et al. paper entitled "Ion-Implanted Microwave Field Effect Transistors in GaAs", Solid State Electronics, Vol. 18, pages 349-353.
With the transistor of the construction described above, for the purpose of recovering distorted crystal structure caused by the implanted ions which are implanted into the semiconductor substrate for forming the semiconductor layer and of electrically activating the implanted ions of an impurity it is necessary to subject the implanted substrate to an annealing treatment in which the substrate is heated to a high temperature of 800.degree. to 900.degree. C. However, such annealing treatment encounters the following problem. More particularly, such residual impurities as chromium, silicon, etc. contained in the substrate at the time of preparing the semiinsulating semiconductor substrate tend to diffuse or unwanted external impurities might be incorporated, or impurities implanted into a predetermined portion at a predetermined concentration tend to diffuse. In addition, when the surface of the semiinsulating semiconductor substrate is subjected to the high temperature described above, such surface of compound semiconductor as GaAs often decomposes. Due to various phenomena appearing at the time of annealing it is difficult to obtain at a high reproducibility a semiconductor having a uniform thickness and containing an impurity at a uniform concentration. This also causes dispersion in the pinch off voltage of the resulting transistor thus decreasing the yield.